src: Refactoring common.hpp and minimizing exposed includes

Face class has relatively large rewrite to hide internals using
Implementor pattern.  As of this commit, <boost/asio.hpp> is not
automatically included whenever ndn-cxx/face.hpp is included.  If it is
needed to directly work with io_service, asio.hpp should be specifically
included.

Change-Id: Ie742b851025b4e3da634eb981319df0f42937855
diff --git a/src/util/config-file.hpp b/src/util/config-file.hpp
index 8d49d26..880657a 100644
--- a/src/util/config-file.hpp
+++ b/src/util/config-file.hpp
@@ -15,6 +15,8 @@
 
 #include "../common.hpp"
 
+#include <fstream>
+
 #include <boost/property_tree/ptree.hpp>
 #include <boost/filesystem.hpp>